As translation of mRNA proceeds, stable complexes are formed. These complexes are made of ribosomes bound to mRNA with tRNA and nascent polypeptide encoded by the messenger RNA. Termed "ribosome complexes" herein, such complexes can be isolated by various known processes (Connolly and Gilmore (1986) J. Cell Biol. 103:2253; Perara et al. (1986) Science 232:348). Antigen-encoding mRNAs have been purified by taking advantage of the immunoreactivity of nascent polypeptides associated with ribosome complexes (Sambrook, J., Fritsch, E. F., Maniatis, T. Molecular Cloning: A Laboratory Manual (Cold Spring Harbor, N.Y.) (1989) ibid. sections 8.9-8.10). Such immunoreactive ribosome complexes can be immunoprecipitated from solution or separated by protein A column chromatography from non-reactive ribosome complexes (Schutz et al. (1977) Nuc. Acids Res. 4, 71; Shapiro and Young (1981) J. Biol. Chem. 256, 1495). Cyclical selection and amplification of RNAs with partitionable properties is now also possible. Historically, mRNA selection is closely tied to immunopurification of ribosome complexes, however, the partitioning of ribosome complexes according to the present invention is not restricted to immunoreactivity of the nascent polypeptides.